The present invention relates to a method for forming a composite rod of acrylic resin, said rod comprising a colorless or clear central portion and a peripheral surrounding portion of a darker color. The composite rods made according to this invention can be used to form single-piece intraocular lenses (IOL's) having a central lens body and colored positioning loops.
Single-piece IOLs, commonly fabricated of the polymer polymethylmethacrylate (PMMA), are widely used for implantation in both anterior and posterior chambers of the eye. The IOLs commonly comprise a central lens body having positioning loops extending radially therefrom. It having been recognized that IOLs which are fabricated entirely of clear, colorless material can be difficult to visualize and manipulate during implantation, there have been suggestions to form the positioning loops of a colored material. Numerous IOLs having separate, colored positioning loops have been marketed. Methods for fabricating single-piece IOLs having colored positioning loops have also been suggested.
U.S. Pat. No. 4,813,956, issued to Gupta on Mar.21, 1989, discloses a method cf forming single-piece intraocular lenses comprising the steps of forming a thin sheet of colored polymethyl methacrylate, coring the sheet to form holes therein, filling the holes with a clear or differently colored PMMA material, polymerizing the colored and clear or differently colored PMMA material comprising the sheet and filled holes, cutting core members from the polymerized sheet each having an inner circular region of PMMA material and an outer region of colored PMMA material and machining a single-piece intraocular lens from a core member to have a central lens body of PMMA material and colored PMMA positioning loops extending from and integral with the central lens body.
U.S. Pat. Nos. 4,676,791 and 4,774,036, issued to LeMaster et al. on June 30, 1987, and Sept. 27, 1988, respectively, disclose IOLs with color ringed or rimmed edges. The lenses can be fabricated by passing a clear rod of optical quality PMMA through an extrusion orifice and coating the circumference of the rod with a layer of colored PMMA or other compatible material. Other methods of fabrication can include the introduction of a suitable dye into the outer regions of the rod, or joining the clear central region to a ring of colored material by thermal or adhesive bonding or other known processes. By lathing loops from the colored portion of the rod and lathing the optic from the clear portion of the rod, there is manufactured a single piece lens with colored loops and partially colored optic.
U.S. Pat. No. 4,687,485, issued to Lim et al. on Aug. 18, 1987, discloses an intraocular lens having colored positioning legs. The patent suggests that such lenses may be made by forming a rod of an appropriate polymer, such as polymethylmethacrylate, centering the rod in a tube, pouring into the tube a solution of initiator, monomer and dye, and allowing the solution to polymerize. The resultant two-layered rod can be cut into discs which can be lathe cut and machined to form an IOL.
U.S. Pat. No. 3,619,004, issued to Kamath on Nov. 9, 1971, discloses contact lenses having an edge of a plastic material which is softer than the poly (4-methyl pentene-1) from which the remainder of the lens is formed. The patent suggests that the lens may be prepared by forming a rod of poly (4-methyl pentene-1), submerging the rod in a vessel containing an aqueous solution of acrylic acid and a homopolymerization inhibitor, subjecting the submerged rod to gamma radiation to yield a rod grafted with a swollen polyacrylic acid polymer cladding. Buttons may be cut from the composite rod, and lenses machined from the buttons.
The methods heretofore taught for preparing composite rods from which IOLs with colored positioning loops may be machined have not been entirely satisfactory. Acrylic polymers such as PMMA, the material of choice for IOLs, swell in the presence of acrylic monomers. Thus, when one surrounds a PMMA rod with a mixture of MMA monomer, initiator, and dye (with the intention of then polymerizing the MMA/dye to form a colored peripheral portion), the PMMA rod can swell up to about two and one half times its normal size. In so doing, of course, the MMA and dye penetrate the PMMA rod so that it is not possible to obtain a final composite rod having two distinct regions, a clear central region and a peripheral region of a different color.